The present invention relates to data acquisition, and more particularly to a multiple probe acquisition system for obtaining synchronized data from self-contained acquisition probes.
In a conventional test and measurement system a probe is used to route an electrical signal via a cable from a circuit under test to an instrument. The instrument, such as an oscilloscope or logic analyzer, includes an acquisition circuit and a trigger circuit to which the cable from the probe is connected. In the acquisition circuit the electrical signal is sampled and digitized, with the resulting data being stored for further processing in an acquisition memory in response to a trigger signal from the trigger circuit. The processed data is then presented as a waveform and/or as alphanumeric information on a display. There are generally two types of probes—passive probes and active probes.
Passive probes rely upon power from the circuit under test to drive the electrical signal from the probe to the acquisition circuit in the instrument over the cable. These probes are relatively inexpensive and are used almost exclusively by value or lower performance instruments. However passive probes present a load to the circuit under test which distorts the electrical signal acquired by the instrument. Further the cable presents an impedance path between the probe and the acquisition circuit which further may distort the electrical signal acquired by the instrument. Finally the cable presents a bandwidth limitation on the signals that may be accurately passed to the instrument. To address this problem passive probes are designed with very low capacitance and the cable is kept as short as possible, but this does not eliminate completely the distortion problem.
Active probes deliver a more accurate waveform display than passive probes, even at the lower frequencies of value instruments. There are two basic reasons for this increase in accuracy, both related to reducing the capacitance of the probe. As indicated above the probe capacitance loads the circuit under test, causing waveform distortion. The probe capacitance also causes alternating current to pass from the circuit under test to the ground lead of the probe. The current in the ground lead then produces a voltage across the ground lead, and inductance in the ground path causes a voltage along the ground lead. The resulting voltage displayed as the waveform is the signal being monitored minus the induced voltage in the ground lead. To reduce the distorting effects of the ground lead, either the ground lead is shortened to reduce the inductance or the probe capacitance is reduced. In many circumstances there are not convenient ways to make low inductance ground paths.
For a high performance instrument active probes have been developed that derive power from the instrument to drive the electrical signal from the probe to the acquisition circuit in the instrument. These probes may include a programmable amplifier and attenuator which receive power and commands from the instrument. The active probes are relatively expensive compared to the passive probes because of the added electronic circuitry, but are small in cost compared to the expense of the high performance instrument. The active probes also are more difficult to use than the passive probes. The reason for more cost is the fact that power, signaling and very high bandwidth signals are passed through the cable. Additionally, the active probes are designed for operation at bandwidths much above those needed for value instruments. Further, although the active probe minimizes loading of the circuit under test, the cable is still a source of signal distortion between the probe and the acquisition circuit in the instrument, especially at high frequencies in the GigaHertz range. Additionally at one watt of power the probe becomes hot, which is not desirable to a user when probing the circuit under test, so the amount of electronics in the active probe is kept to a minimum. Finally most active probes are limited in the voltage range that can be probed as they do not contain attenuators that are controlled by the instrument's volts/division control, and so are not useful for general purpose applications. When an inexperienced user views a signal with too much amplitude, the probe delivers a clipped signal that is displayed as an extremely distorted signal that may fool the user into believing that it is the actual signal present at the probe tip.
For value or lower performance instruments active probes are not generally used because the cost of the active probe is comparable to the cost of the value instrument. Therefore value instruments generally use only passive probes for acquiring signals from the circuit under test.
Recently there have been new products that seek to replace the stand alone low value oscilloscopes. These products are adapted to interact with a personal computer (PC), and therefore include some acquisition circuitry as part of the probe system. One such product is represented by a pen-type single channel digital storage oscilloscope (DSO), such as the PenScope osziFOX® DSO manufactured by Wittig Technologies and the PicoScope® USB Pen Scope manufactured by Pico Technology Limited, that uses a serial interface cable to display recorded signals on the PC screen. Another such product is a USB module to which probes are connected, such as the USBee ZX Digital Test Pod manufactured by CWAV or the MS-32 Oscilloscope Mixed Signal Option manufactured by LeCroy Corporation, which performs triggering and data acquisition and communicates with the PC or a test instrument.
When multiple probes are grouped together to monitor different portions of the circuit under test, each probe has its own trigger circuit. However there is no system that allows synchronization of data from the multiple probes so that a user can see what is happening at each probe location when a trigger event occurs. In otherwords, each probe system operates independently of any other probe systems.
What is desired is a probe system for lower performance, value instruments that is general purpose, i.e., good dynamic range controlled from the instrument, low cost and less capacitance to minimize loading of the circuit under test, and that is able to synchronize data acquisition with other probe systems.